Anomalous Hall effect in granular ferromagnetic metals and effects of weak localization

We theoretically investigate the anomalous Hall effect in a system of dense-packed ferromagnetic grains in the metallic regime. Using the formalism recently developed for the conventional Hall effect in granular metals, we calculate the residual anomalous Hall conductivity $\sigma_{xy}$ and resistivity $\rho_{xy}$ and weak localization corrections to them for both skew-scattering and side-jump mechanisms. We find that, unlike for homogeneously disordered metals, the scaling relation between $\rho_{xy}$ and the longitudinal resistivity $\rho_{xx}$ does not hold. The weak localization corrections, however, are found to be in agreement with those for homogeneous metals. We discuss recent experimental data on the anomalous Hall effect in polycrystalline iron films in view of the obtained results.Comment: published version, 10 pages, 6 figure

Enhancement of Superconductivity in Disordered Films by Parallel Magnetic Field

We show that the superconducting transition temperature T_c(H) of a very thin highly disordered film with strong spin-orbital scattering can be increased by parallel magnetic field H. This effect is due to polarization of magnetic impurity spins which reduces the full exchange scattering rate of electrons; the largest effect is predicted for spin-1/2 impurities. Moreover, for some range of magnetic impurity concentrations the phenomenon of {\it superconductivity induced by magnetic field} is predicted: superconducting transition temperature T_c(H) is found to be nonzero in the range of magnetic fields $0 < H^* <= H <= H_c$.Comment: 4 pages, 2 figure

ЦИКЛОКОНВЕРТОР С КОМБИНИРОВАННЫМ ЗАКОНОМ УПРАВЛЕНИЯ ДЛЯ СИСТЕМ ЭЛЕКТРОСНАБЖЕНИЯ АВТОНОМНЫХ ОБЪЕКТОВ

The present paper considers cycle-converter with complicated control law. The methodj complicated control law generating is presented. The scheme of power circuits is introduced. Volta and current curves for classical cycle-converter and cycle-converter with complicated control lawi discussed. Introducing of complicated control law improves output voltage quality, increa fundamental harmonic's amplitude and input power factor. The article is attractive in the field of i power electronics.В статье рассматривается циклоконвертер с комбинированным законом управления вентильными комплектами. Представлен способ формирования комбинированного сигнала управления. Приведена схема силовых цепей исследуемого преобразователя. Рассматриваются эпюры кривых выходного напряжения и тока для классической схемы циклоконвертера и для схемы преобразователя с комбинированным законом управления. Введение комбинированного сигнала управления повышает качество выходного напряжения, увеличивает амплитуду основной гармоники и входной коэффициент мощности. Статья представляет интерес для специалистов в области авиационной силовой электроники.В статье рассматривается циклоконвертер с комбинированным законом управления вентильными комплектами. Представлен способ формирования комбинированного сигнала управления. Приведена схема силовых цепей исследуемого преобразователя. Рассматриваются эпюры кривых выходного напряжения и тока для классической схемы циклоконвертера и для схемы преобразователя с комбинированным законом управления. Введение комбинированного сигнала управления повышает качество выходного напряжения, увеличивает амплитуду основной гармоники и входной коэффициент мощности. Статья представляет интерес для специалистов в области авиационной силовой электроники

Surface impedance of superconductors with magnetic impurities

Motivated by the problem of the residual surface resistance of the superconducting radio-frequency (SRF) cavities, we develop a microscopic theory of the surface impedance of s-wave superconductors with magnetic impurities. We analytically calculate the current response function and surface impedance for a sample with spatially uniform distribution of impurities, treating magnetic impurities in the framework of the Shiba theory. The obtained general expressions hold in a wide range of parameter values, such as temperature, frequency, mean free path, and exchange coupling strength. This generality, on the one hand, allows for direct numerical implementation of our results to describe experimental systems (SRF cavities, superconducting qubits) under various practically relevant conditions. On the other hand, explicit analytical expressions can be obtained in a number of limiting cases, which makes possible further theoretical investigation of certain regimes. As a feature of key relevance to SRF cavities, we show that in the regime of "gapless superconductivity" the surface resistance exhibits saturation at zero temperature. Our theory thus explicitly demonstrates that magnetic impurities, presumably contained in the oxide surface layer of the SRF cavities, provide a microscopic mechanism for the residual resistance.Comment: 9 pages, 3 figs; v2: published versio

Intracoronary administration of metabolic agents in the treatment of acute coronary syndrome

The article presents the new possibilities of preventing the onset of irreversible myocardial damage during ischemia and effective approaches undertaken in the early reperfusion period (per - and postconditioning of the myocardium) through coronary the introduction of cardioprotective drugs. Practitioners need medicines, which have fewer complications and are universal this is by far the metabolic drugs. The author surveyed 300 patients (three groups) with acute coronary syndrome (ACS) hospitalized no later than 4 hours from the beginning of anginal attack. Patients in group I (n=109) in the prehospital phase was carried out by the system tromboliticescoy therapy (TLT) in combination with intravenous etilmetilgidroksipiridina succinate (200 mg), which the hospital performed mechanical recanalization and angioplasty of the infarct-related coronary artery (IRA) with intracoronary introduction of etilmetilgidroksipiridina succinate (200 mg). Patients in group II (n=101) etilmetilgidroksipiridina succinate (200 mg) primary intracoronary injected during the endovascular procedure. Patients III (control) group (n=90) was carried out only angioplasty of the IRA. Conclusions: the effect of restoration of coronary circulation set after conducted thrombolytic therapy with intracoronary administration of etilmetilgidroksipiridina succinate immediately after successful angioplasty of the IRA, which helped to restore adequate blood flow to prevent relapses and extension of the necrosis zone, transition to a higher angina functional classes

Hall Transport in Granular Metals and Effects of Coulomb Interactions

We present a theory of Hall effect in granular systems at large tunneling conductance $g_{T}\gg 1$. Hall transport is essentially determined by the intragrain electron dynamics, which, as we find using the Kubo formula and diagrammatic technique, can be described by nonzero diffusion modes inside the grains. We show that in the absence of Coulomb interaction the Hall resistivity $\rho_{xy}$ depends neither on the tunneling conductance nor on the intragrain disorder and is given by the classical formula $\rho_{xy}=H/(n^* e c)$, where $n^*$ differs from the carrier density $n$ inside the grains by a numerical coefficient determined by the shape of the grains and type of granular lattice. Further, we study the effects of Coulomb interactions by calculating first-order in $1/g_T$ corrections and find that (i) in a wide range of temperatures T \gtrsim \Ga exceeding the tunneling escape rate \Ga, the Hall resistivity $\rho_{xy}$ and conductivity \sig_{xy} acquire logarithmic in $T$ corrections, which are of local origin and absent in homogeneously disordered metals; (ii) large-scale ``Altshuler-Aronov'' correction to \sig_{xy}, relevant at T\ll\Ga, vanishes in agreement with the theory of homogeneously disordered metals.Comment: 29 pages, 16 figure

Excitonic condensation in a double-layer graphene system

The possibility of excitonic condensation in a recently proposed electrically biased double-layer graphene system is studied theoretically. The main emphasis is put on obtaining a reliable analytical estimate for the transition temperature into the excitonic state. As in a double-layer graphene system the total number of fermionic "flavors" is equal to N=8 due to two projections of spin, two valleys, and two layers, the large-$N$ approximation appears to be especially suitable for theoretical investigation of the system. On the other hand, the large number of flavors makes screening of the bare Coulomb interactions very efficient, which, together with the suppression of backscattering in graphene, leads to an extremely low energy of the excitonic condensation. It is shown that the effect of screening on the excitonic pairing is just as strong in the excitonic state as it is in the normal state. As a result, the value of the excitonic gap \De is found to be in full agreement with the previously obtained estimate for the mean-field transition temperature $T_c$, the maximum possible value $\Delta^{\rm max},T_c^{\rm max}\sim 10^{-7} \epsilon_F$ ($\epsilon_F$ is the Fermi energy) of both being in $1{\rm mK}$ range for a perfectly clean system. This proves that the energy scale $\sim 10^{-7} \epsilon_F$ really sets the upper bound for the transition temperature and invalidates the recently expressed conjecture about the high-temperature first-order transition into the excitonic state. These findings suggest that, unfortunately, the excitonic condensation in graphene double-layers can hardly be realized experimentally.Comment: 21 pages, 5 figures, invited paper to Graphene special issue in Semiconductor Science and Technolog

Excitonic condensation in a double-layer graphene system

The possibility of excitonic condensation in a recently proposed electrically biased double-layer graphene system is studied theoretically. The main emphasis is put on obtaining a reliable analytical estimate for the transition temperature into the excitonic state. As in a double-layer graphene system the total number of fermionic "flavors" is equal to N=8 due to two projections of spin, two valleys, and two layers, the large-$N$ approximation appears to be especially suitable for theoretical investigation of the system. On the other hand, the large number of flavors makes screening of the bare Coulomb interactions very efficient, which, together with the suppression of backscattering in graphene, leads to an extremely low energy of the excitonic condensation. It is shown that the effect of screening on the excitonic pairing is just as strong in the excitonic state as it is in the normal state. As a result, the value of the excitonic gap \De is found to be in full agreement with the previously obtained estimate for the mean-field transition temperature $T_c$, the maximum possible value $\Delta^{\rm max},T_c^{\rm max}\sim 10^{-7} \epsilon_F$ ($\epsilon_F$ is the Fermi energy) of both being in $1{\rm mK}$ range for a perfectly clean system. This proves that the energy scale $\sim 10^{-7} \epsilon_F$ really sets the upper bound for the transition temperature and invalidates the recently expressed conjecture about the high-temperature first-order transition into the excitonic state. These findings suggest that, unfortunately, the excitonic condensation in graphene double-layers can hardly be realized experimentally.Comment: 21 pages, 5 figures, invited paper to Graphene special issue in Semiconductor Science and Technolog

Excitonic condensation in a double-layer graphene system

The possibility of excitonic condensation in a recently proposed electrically biased double-layer graphene system is studied theoretically. The main emphasis is put on obtaining a reliable analytical estimate for the transition temperature into the excitonic state. As in a double-layer graphene system the total number of fermionic "flavors" is equal to N=8 due to two projections of spin, two valleys, and two layers, the large-$N$ approximation appears to be especially suitable for theoretical investigation of the system. On the other hand, the large number of flavors makes screening of the bare Coulomb interactions very efficient, which, together with the suppression of backscattering in graphene, leads to an extremely low energy of the excitonic condensation. It is shown that the effect of screening on the excitonic pairing is just as strong in the excitonic state as it is in the normal state. As a result, the value of the excitonic gap \De is found to be in full agreement with the previously obtained estimate for the mean-field transition temperature $T_c$, the maximum possible value $\Delta^{\rm max},T_c^{\rm max}\sim 10^{-7} \epsilon_F$ ($\epsilon_F$ is the Fermi energy) of both being in $1{\rm mK}$ range for a perfectly clean system. This proves that the energy scale $\sim 10^{-7} \epsilon_F$ really sets the upper bound for the transition temperature and invalidates the recently expressed conjecture about the high-temperature first-order transition into the excitonic state. These findings suggest that, unfortunately, the excitonic condensation in graphene double-layers can hardly be realized experimentally.Comment: 21 pages, 5 figures, invited paper to Graphene special issue in Semiconductor Science and Technolog